Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.
In general, fuel cell technology includes a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into several general categories, namely: (i) fuel cells utilizing compressed hydrogen (H2) as fuel; (ii) proton exchange membrane (PEM) fuel cells that use alcohols, e.g., methanol (CH3OH), metal hydrides, e.g., sodium borohydride (NaBH4), hydrocarbons, or other fuels reformed into hydrogen fuel; (iii) PEM fuel cells that can consume non-hydrogen fuel directly or direct oxidation fuel cells; and (iv) solid oxide fuel cells (SOFC) that directly convert hydrocarbon fuels to electricity at high temperature.
Compressed hydrogen is generally kept under high pressure and is therefore difficult to handle. Furthermore, large storage tanks are typically required and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. The most common direct oxidation fuel cells are direct methanol fuel cells or DMFC. Other direct oxidation fuel cells include direct ethanol fuel cells and direct tetramethyl orthocarbonate fuel cells. DMFC, in which methanol is reacted directly with oxidant in the fuel cell, is the simplest and potentially smallest fuel cell. It also has promising power application for consumer electronic devices. SOFC convert hydrocarbon fuels, such as butane, at high heat to produce electricity. SOFC requires relatively high temperature in the range of 1000° C. for the fuel cell reaction to occur.
The chemical reactions that produce electricity are different for each type of fuel cell. For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half-reaction at the cathode:1.5O2+6H++6e−→3H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to both the migration of the hydrogen ions (H+) through the PEM from the anode to the cathode and the inability of the free electrons (e−) to pass through the PEM, the electrons flow through an external circuit, thereby producing an electrical current. The external circuit may be used to power many useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers, and power tools, among others.
DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated herein by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.5 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
In another direct oxidation fuel cell, borohydride fuel cell (DBFC) reacts as follows:
Half-reaction at the anode:BH4−+8OH−→BO2−+6H2O+8e−
Half-reaction at the cathode:2O2+4H2O+8e−→8OH−
In a chemical metal hydride fuel cell, generally aqueous sodium borohydride is reformed and reacts as follows:NaBH4+2H2O→(heat or catalyst)→4(H2)+(NaBO2)
Half-reaction at the anode:H2→2H++2e−
Half-reaction at the cathode:2(2H++2e−)+O2→2H2O
Suitable catalysts for this reaction include platinum and ruthenium, as well as other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. A sodium borate (NaBO2) byproduct is also produced by the reforming process. A sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated herein by reference. Therefore, the known chemical hydride reactions that use aqueous metal hydride have about 9 to 12 weight percentage storage expectancy, and the liquid and the catalyst used in the wet chemical reaction system need to be closely monitored. Additionally, it is difficult to maintain the stability of a metal hydride solution over a long period of time, because according to the formula t1/2−pH*log(0.034+kT), which provides the half life of the reaction, the reaction of hydrolysis always occurs very slowly. Furthermore, if the solution is stabilized, the reactivity is not complete.
In a hydride storage method, the reaction is as follows:Metal+H2→hydride+heat
However, storage expectancy of such a reaction is only about 5 weight percentage. Additionally, such reactions can be expensive and difficult to package.
Another known method to produce hydrogen is a dry hydride reaction. Dry reaction, generally, involves the following reaction:X(BH4)→H2,where X includes, but is not limited to, Na, Mg, Li, etc.
Again, dry reactions have several disadvantages, such as having a storage expectancy of only about 10 weight percentage, and the need to closely monitor the pressure.
An additional method to produce hydrogen gas is by a pressure storage method using the formula PV=nRT, wherein P is pressure, V is volume, n is a number of moles, R is the gas constant, and T is temperature. This method requires constant pressure monitoring.
One of the most important features for fuel cell application is fuel storage. Another important feature is regulating the transport of fuel out of the fuel cartridge to the fuel cell. To be commercially useful, fuel cells such as DMFC or PEM systems should have the capability of storing sufficient fuel to satisfy the consumers' normal usage. For example, for mobile or cell phones, for notebook computers, and for personal digital assistants (PDAs), fuel cells need to power these devices for at least as long as the current batteries and, preferably, much longer. Additionally, the fuel cells should have easily replaceable or refillable fuel tanks to minimize or obviate the need for lengthy recharges required by today's rechargeable batteries.
One common disadvantage of the known hydrogen gas generators is that once the reaction starts, the gas generator cartridge cannot control the reaction. Thus, the reaction will continue until the supply of the reactants run out or the source of the reactant is manually shut down. Moreover, the known hydrogen generators may not work when positioned in a certain orientation and are relatively large in size. Accordingly, there remains a need for improved hydrogen generators and the reactants producing hydrogen.